The facsimile recorder has been known and used extensively which employs a roll of thermal transfer type recording sheet, known as a donor roll, through which the subject copy is thermally transferred to paper. Some facsimile recorders on the market are further equipped to detect the fact that the roll is being used up, and to visually or audibly forewarn the user of the end of the roll.
In order to make possible the automatic detection of the fact that the recording sheet is coming near to its end, it has been practiced to provide an end mark having a reflective surface on the sheet, in a position spaced a prescribed distance from its end anchored to the roll shaft. The end mark may be formed either directly on one side of the base film of the recording sheet or on the hot-melt ink layer on the other side off the base film. The end mark is optically detected by a sensor comprising a source of infrared radiation and a photodetector responsive to such radiation. Flexography and brushing represent two typical conventional measures for creating such end marks.
Such conventional methods of forming end marks are objectionable for several reasons. First, for flexographic production of end marks, the printing ink of reflective material is pressed against the recording sheet by a rubber-made relief plate. The recording sheet is so thin, however, that the relief plate tends to wrinkle the sheet when pressed against the same via the reflective material. At the same time, moreover, the reflective ink easily oozes out from between the recording sheet and the plate, thereby forming undesired bulges beyond the due boundaries of the end marks. Such bulges not only blur the bounding edges of the end marks but also make their thickness uneven.
Additional disadvantages of flexography arise from the fact that before printing, the reflective ink on the rubber plate is in the form of a film overlying the protuberane parts of the relief plate. The ink film tends to developed unevenness on its transfer from the plate to the recording sheet, and it is difficult to control the amount of the ink so transferred and, therefore, the thickness of the end mark so printed. The composition of the ink is also subject to the restriction that it should contain no such solvent as will attack the rubber plate. This restriction imposes additional limitations on the choice of resins to be contained in the ink as a binder. Accordingly, the desired dispersion characteristics of the pigment or powdered metal contained in the ink are not easy to realize, with a consequent decrease in the quality of the printings. It is a still further weakness of flexography that the rubber plates are susceptible to deformation and poor in durability.
The production of end marks by brushing is also objectionable because of the poor quality of the markings so produced. What is worse, this conventional method is very time-consuming and not suitable for mechanized production of the recording sheets on a large scale.
For the reasons set forth in the foregoing, the end marks produced in accordance with the prior art, either by flexography or by brushing, have often been of uneven thickness and have not been defined clearly enough. Such defective end marks have often invited misdetection by infrared sensors, with the result that no warning is generated at the required time before the roll of recording sheet is used up.
Known apparatuses for the manufacture of rolls of recording sheets with end marks thereon have also had difficulties in connection with the application of end marks. Heretofore, the end marks have been formed off-line in longitudinally spaced groups on a continuous web of base film with a hot-melt ink layer thereon. The web has a width several times greater than that of each strip of recording sheet to be produced; and each group of end marks are arranged side by side in the transverse direction of the web. Then, by a separate apparatus, the web has been slit longitudinally into the required narrower strips of recording each bearing an end mark thereon.
An objection to this conventional method of manufacture is that if the end-marked web wrinkles or developes other defects while being slit, one roll length of the web has to be discarded. Another weakness is the inefficiency of production due to the fact that the application of end marks to the web and the slitting of the end-marked web require separate machines totally disconnected from each other.
Accordingly, it is an object of the invention to provide a roll or strip of thermal transfer type recording sheet having an end mark so formed as to assure infallible detection by an optical sensor in order to warn the user of the approaching end or the strip.
Another object of the invention is to provide an apparatus for efficient manufacture of such a roll or strip of recording sheet having the end mark.